Control system and process for refining metals



1963 1.. VON BOGDANDY ET A 3,100,599

CONTROL SYSTEM AND PROCESS FOR REFINING METALS Filed Sept. 12, 1960 Lizoifii mw omhzou 3,100,699 CONTROL SYSTEM AND PROiJESS FOR REFHNING METALS Ludwig von Bogdandy, Esseu-Frintrop, and Heinz-Dieter Our present invention relates to a control system for Patented Aug. 13, 1963 the example given), less the inherent response lag of the system whereby the feedback becomes effective in a subsequent cycle in exactly the proper phase with the changes I in the interfering factor (the. effective bath level) which stabilizing, through feedback, an output variable subject to both long-term and short-term variations, the latter variations being due to a rhythmically recurring but otherwise unpredictable interference factor and being therefore of an essentially periodic nature. I

A typical system of this character is a rotary refimng furnace for oarbonacontaining metals in which the carbon contained in the melt is first oxidized, with the aid of a submerged or oxygen blast, to carbon monoxide which rises above the melt surface and is there further oxidized, by means of another such blast, to carbon dioxide. The latter reaction, occurring within the atmosphere of the'furnace as it. rotates about a generally horizontal axis, contributes to the heating of the melt and so increases the thermal efficiency of the apparatus. For optimum performance it is, of course, desirable that the admission of oxygen both below and above the bath surface be so dosed as to result in a substantially complete combustion of all the developing carbon monoxide without, however, leaving an appreciable excess of oxygen in the waste gases. It has been found that the rate of carbonmonoxide release is a function of the depth at which the primary oxygen is introduced below the bath surface;

control of this depth, therefore, affords arelatively simple means for rapidly varying the rate of carbon monoxide generation, in response to the detection of excess CO or 0 in the waste gases, in order to restore the proper balance between the two reagents in the furnace atmos phere. As the furnace rotates, however, the bath level fluctuates periodically as a result of slight eccentricities of the furnace wall and/or surface irregularities thereon dueto uneven wear; These unavoidable fluctuations, which change the effective depth of the submerged blast, :superimpose upon the output variable, is; upon the measured waste-gas composition, a pattern of rhythmic variations which must also be compensated by the feedback control if the desired stability is to be maintained.

The compensation of these short-term rhythmic variations is, however, complicated by the'inherent response lag of such systems, ie by the interval which must elapse between any adjustment of the input variable (such asthe are to be compensated.

More specifically, the invention proposes the recording of the aforementioned short-term fluctuations in a first operating cycle, the utilization of these recordings with a time lead equal to the inherent response lag in a second cycle along with the recording of the residual shortterm fluctuations in that cycle, and analogous utilization of all such recordings in subsequent cycles to cause a progressive reduction of the recorded deviations. The actual magnitude of the input variable (i.e. the absolute location of the submerged blast) is recorded concurrently, during the second and-subsequent cycles, as a basic feedback signal, reproduced without phase shift, upon which the corrective values from the first-mentioned recordings are superimposed.

A practical system for realizing the method described above resides in the provision of an endless recording medium, such as an annular magnetic tape, which is rotated in strict synchronism with the changes in the interfering factor so that each of its revolutions corresponds to exactly one period of the short-term fluctuations due to such factor. The desired phase shift, corresponding to a full period less the inherent lag, is then conveniently obtained by positioning a recording device and a reading device, such as a pair of magnetic heads, in spaced-apart relationship along the circumference of the recording medium so that the recorded signals will reach the reading device after traveling substantially less than a full circle. The storing and the pickup of the feedback sig nals may be concurrently accomplished by means of a second recording medium, rotating at the same speed as the first one, on which the recording and reading devices are positioned close together so that reading takes place substantially a full period after recording. .In a refining system as described above, the recording media may be carried on drums, rigidly coupled with the rotating furnace or otherwise be synchronized with the motion of the latter.

It will be apparent that, with the arrangement just deif, however, this period is not constant, asin the case of a 7 this phase lead in order that the absolute advance of the V depth of the submerged blast in the illustrative example 7 given above) and the resulting change in the output variable to be stabilized (thus, the composition of the waste gases as measured by the analyzer). This inherent lag causes the feedback to operate with a phase displacement which reduces its effectiveness and may even be of such magnitude as to prevent proper control. Our invention, accordingly, has for its principal object the provision of a method of eliminating this drawback as well as a system for putting this method into practice. I

A more specific object of our invention is to provide a system for controlling the operation of a refining furnace, of the character described above, to obviate the disadvantages set forth. Fundamentally, the invention involves the storing of control signals, derived from the output variable to be refining furnace whose rotary speed is subject to variatrons, it will. be desirable for proper functioning to vary feedback within each operating cycle remain constant and equal to the response lag which is generally independent of the aforementioned fluctuation period. Another object of our invention, therefore, is to provide means for conveniently varying the effective distance between the recording and reading devices to insure substantial constaucy of the transit time which the recording medium requires in passing from the reading point to the recording point in the completion of a cycle, this transit time representing the absolute advance of the timing of the feedback provided in accordance 'with'our invention.

A more particular feature of our invention, designed to realize the last-mentioned object, is the provision of guide means for the recording medium so coupled with the controlled system as to draw the recording medium, at a location between the recording and reading devices, into a loop Whose length varies with the speed of the recording medium to maintain the aforementioned transit time constant. If the direction of the controlled system and, therefore, of the recording medium is reversible,the recording device may be flanked on opposite sides by The embodiment of our invention shown in the draw-.

ing is centered on a generally cylindrical furnace 1 rotatably lodged on supports generally indicated at 31, 32. A bath 33, consisting of molten iron to be refined, occupies the lower portion of the furnace 1 underneath its trunnions through which primary oxygen, supplied below the bath surface, and secondary oxygen, introduced above that surface, enters via a vertically movable pipe 34 and a stationary pipe 35, respectively. Waste gases from the interior of the furnace are collected by a hood 36 and conducted toward a gas analyzer 3.

The vertical position of the inlet pipe 34, and therefore the depth to which the oxygen nozzles thereof are submerged within the bath 33, is controlled by a reversible electromotor 9 driving a lead screw 37 which co -acts with a nut 38 secured to the pipe 34. Another motor 2, reversibly energized from a source not shown, drives the furnace 1 through gears 39, 40 and is synchronized, by mechanical or electrical means diagrammatically indicated at 14, with a further motor 13 driving two sprocket wheels 41, 42. Each sprocket wheel positively engages a suitably perforated magnetic tape 24 and 25, respectively, each tape being so dimensioned as to perform a full revolution with each revolution of furnace 1. Tape 25 also passes around a stationary idler roller 43; the corresponding roller 44 of tape 24 is displaceably tensioned, by a spring 45, against a stationary anchor 46 so that this tape, passing around idler rollers 47 and to guide rollers 48,

48 at the ends of a bifurcate arm 49, can be stretched into loops of variable length between a recording head 22 and two reading heads 23, 23 equispaced from it on opposite sides. Arm 49 is provided with rack teeth engaged by a pinion 50 whose position is controlled by a servo motor 20. The shaft 51 of pinion 50 also carries a potentiometer 21' which sends a feedback signal to servo motor 20 via an amplifier 19, this amplifier also receiving a control signal from a tachometer 17, which is coupled with motor 13, through an adjustable imped ance 18. The loop-controlling circuit 17-21 is so designed that, for the reasonsindicated above, the transit time of the tape 24 from the operatively connected reading head (here the head 23) to the recording head remains substantially constant with changes in the speed of motors 2 and 13.

The output of gas analyzer 3, which measures the excess of carbon monoxide or oxygen in the waste gases escaping from furnace 1, is transmitted (e.g. as modulation of an A.-C. carrier) to an isolating transformer 4 and thence, on the one hand, to the recording head 22 and, on the other hand, to a differentiation circuit and a transmission line 30 in parallel. Similarly, the position of pipe 34 is translated into an appropriate electrical signal, e.g. with the aid of potentiometer as diagrammatically shown in the drawing, which is then fed to an isolating transformer 11 for delivery to a differentiation circuit 12 and to a recording head 26 associated with magnetic tape 25. Two reading heads 27', 27" are positioned along tape 25 on opposite sides of recording head 26, in' the immediate vicinity of the latter, so asto receive the" recorded signals after substantially a full revolution of the tape. Over switches 28 and 29, which areoperated manually or automatically upon any reversal in the direction of motors 2 and 13 to activate either the heads 23', 27' (as here shown) or the heads 23", 27", the signals picked up by these heads are fed to a mixer 16 in In the operation of the system shown in the drawing we prefer to proceed as follows:

In a first operating cycle, i.e. during a first revolution of furnace 1, the inlet pipe 34 is held stationary and the tapes 24, 25 are blank so that, initially, no signals are delivered to the controller 7 by dilferentiator 12 or mixer 16. Gas analyzer 3, becoming operative after the response lag represented by the delay between the reaction of the carbon with admitted oxygen within the melt and the appearance of the reaction gases in the input of the analyzer, produces an output signal which is impressed by the recording head 22 upon the tape 24 and will generally exhibit a marked cyclic variation of a period equal to a revolution of the furnace. The opening of a circuit breaker 52 at that time also inactivates the circuits 5 and 13 so that the controller 7, under the sole influence of a constant input from source 6, maintains the motor 9 unoperated; differentiation circuit 12 may be similarly disconnected by a circuit breaker 53.

In the second operating cycle, with the head 23' picking up output signals previously stored on the tape 24, mixer 16 energizes the controller 7 to actuate the motor 9 in a manner tending to compensate the periodic fluctuations in the output variable as measured by gas analyzer 3. Thus, an increase in the CO content of the waste gases will lead to a raising of the pipe 34 toward the furnace axis, in order to reduce the development of CO within the melt, whereas the occurrence of excess oxygen inthe analyzer input will be compensated by a lowering of the pipe for the production of increased amounts of carbon monoxide. The compensating movement of pipe 34 is transmitted via recording head 26 to tape 25 in the form of input signals which show the same periodicity as the original output fluctuations. Thelatter fluctuations, now diminished by the compensatory movement of the pipe 34, are concurrently recorded on the tape 24 in lieu of the original recordings made during thefirst cycle.

In the third cycle the input signals from tape 25, collected after a complete revolution of the latter, and the output signals from tape 24, read with a delay equal to one revolution of that tape diminished by the transit time between heads 23' and 22, are combined in mixer 16 to produce a corrected feedback signal which now controlsthe movement of pipe 34 during that cycle. A like pro cedure is followed during each subsequent cycle, the result being a gradual diminution of the periodic fluctuations in the output signal applied to transformer 4.}

The differentiation circuit 5 also performs an integrating function to determine the slope of changes occurring over a relatively long period.

In order to compensate for the long-term changes in gas composition, e.g. those due to the gradual dissipation of the carbon content of the melt 33, circuit breaker 52 is closed to energize the integrator 5 which, designed to detect these long-term changes, act-uates the controller 7 for a compensatory adjustment of the position of pipe 34. It will also be desirable to modify the operation of the controller in accordance with the corrective factor di/ do, where di and do are the difierentials of the input variable and the output variable as measured by potentiometer 10 and analyzer 3 respectively, by applying to the controller the time differential di/dt from circuit 12 and the time dilferential do/dt from circuit 5 with inverse effects as indicated by the plus andminus signs,

respectively; this action, occurring upon the closure of a controller'isactuated .by the time integral and the time differential of a controlled variable as well .as by the instantaneous value thereof, asutilized in the illustrated embodiment, 'arerwell known per se and are sometimes referred to as JP.I.D; systems. The instantaneous value is supplied by line 30. r

.The differential circuit 12 may include suitable storage or delay, means adapted to introduce a time lag corresponding to the inherent response lag whereby the time differential di/dt may be evaluated concurrently with the dilferential do/dt resulting therefrom. a

It will be understood that the system herein disclosed can operate also with discontinuous rather than continuous measuring and recording of the gas composition or other output variable, the magnetictape being in this case replaceable by a succession of individual electromagnetic, capacitive or other storage devices.

We claim: v d

1. A'method of operating a rotary refining furnace comprising the steps of introducing a gas below the level of a melt. in said furnace to produce an exhaust gas of,

a composition which constitutes an output variable subject to periodic fluctuations, said output variable responding with a predetermined la-g to changes in an input variable defining the depth at which said gas is introduced into said melt, substantially continuously measuring said output variable, substantially continuously storing the I measured values of said output variable for an interval equal to one-whole fluctuation period less a time equal to said lag, and substantially continuously changing said input variable at the end of said interval in step with the values so stored and in a sense counteracting said fluctuations, thereby eliminating the effect of said lag.

2. A method of operatinga rotary refining furnace comprising the steps of introducing a gas below the level of a melt in said furnace to produce an exhaust gas of a compositionwhich constitutes an output variable subject to long-term variations and to periodic short-term fluctuations, said output variable responding with a predetermined lag to changes in an input variable defining the depth at which said gas is introduced into said melt, substantially continuously measuring said output variable,

substantially continuously storing the measured values of said output variable for an interval equal to at least one whole fluctuation'period less a time equal to: said lag, producing a delayed control signal from the values so stored, combining said delayed control signal with another control signal proportional tosaid long-term variations,

" and substantially continuously changing said input variable in step with the combined control signals and in a sense counteracting said fluctuations and variations, thereby eliminating the effect of said lag.

3. A method according to claim 2. wherein said input variable is further modified in inverse ratio to the corrective factor represented by thedilferential quotient di/rlo, di and do being the differentials of the input variable and of the output variable, respectively. I

4-. A method of operating a rotary refining furnace comprising the steps of introducing a gas below the level of a melt in said furnace to produce an exhaust gas of a composition whichconstitutes an output variable subject to periodic fluctuations, said output variable responding with a predetermined time lag to changes in an input variable defining the depth at which said gas is introduced into said melt, measuring said output variable during a first fluctuation period with said input variable maintained constant, storing the so measured values of said output variable, for an interval equal to one whole fluctuation period less thana time equal to said lag,

producing at the end of said interval a delayed control signal from the values so stored, changing said input variable in step with said control signal and in a sense counteracting said fluctuations during a second fluctuation period, storing during said second period a feedback signal representative of the changed values of said input variable While continuing tomeasure said output variable,

storing the values thereof for said interval and producing the delayed control signal therefrom, combining during a third flutcation period the delayed control signal with the feedback signal stored for a whole period, changing said input variable during said third period in step with the combined signals and concurrently storing the changed values of said input variable for another whole period, continuing during said third period to measure said output variable, storing the values thereof for sald interval and producing the delayed control signal therefrom, and proceeding in like manner during subsequent periods, thereby eliminating the eifect of said lag.

5. A method according to claim 4 wherein the measured values of said output variable are stored on an endless recording medium making one revolution during each fluctuation period, they delayed control signal being produced by reading the recorded values on said medium at a location ahead of the recording point after less than a full revolution,

6. A method according to claim 5 wherein the transit time of said medium from the reading point to the recording point is maintained substantially constant in the presence of changes in said fluctuation period by varying the effective distance between said reading and recording points along said medium substantially proportionally to the speed of said medium.

7. A method according to claim 5 wherein said feedback signal is stored on a second recording medium revolving in unison with the first-mentioned medium.

8. A method according to claim 6 wherein said eflective distance is varied by a looping of said medium between said points.

9. A control system for a rotating reaction vessel having output means adapted to produce an output signal representative of the progress of a reaction in said vessel and input means adjustable for changing an operating parameter to influence the reaction with an inherent time lag between the adjustment of said input means and a resulting change in said output signal, said output signal being subject to periodic fluctuations caused by the rotation of said vessel, comprising an endless recording medium, drive means for rotating said recording medium in step with said vessel, recording means connected to said output means and positioned at a first location adjacent said medium for impressing thereon said output signal, reading means positioned at a second location adjacent said medium for picking up the recorded signal 4 after a delay equal to a whole revolution less a time tive means coupled with said drive means for displacing said guide means in a manner varying the length of said loop between said locations substantially in proportion to the speed of said tape, thereby maintaining a substantially invariable transit time of said tape from said second to said first location.

#11. A control system according to claim 9, further comprising a second endless recording medium coupled with said drive means for rotation at the same rate as the first-mentioned recording medium, feedback means controlled by said input means for producing a feedback signal representative of said parameter, other recording means connected to said feedback means for storing said feedback signal on said second medium, other reading means positioned alongside said other recording means adjacent said second medium for recovering said feedback signal after storage for substantially one whole revolution, and mixer means for combining the outputs of both of said reading means and applying the combined outputs to said control means.

12. A control system according to claim 9 wherein said vessel is a refining furnace rotatable about a generally horizontal axis, said furnace being provided with an inlet for an oxygen blast terminating at a location below the level of a bath of molten metal in its interior, said input means comprising mechanism for vertically adjusting the position of said inlet within said furnace with respect to said axis; I

13. A method of operating a rotary refining furnace comprising the steps of introducing a gas below the level of a melt in said furnace to produce an exhaust gas of a composition which constitutes an output variable subject to periodic fluctuations, said output variable responding with a predetermined time lag to changes in an input variable defining the depth at which said gas is introduced into said melt, comprising-the steps of substantially continuously measuring said output variable during a first fluctuation period with said input variable maintained constant, substantially continuously storing the so measured values of said output variable for an interval equal a to one whole fluctuation period less than a time equal to said lag, producing at the end of said interval a delayed control signal from the values so stored, substantially continuously changing said input variable in step with said control signal and in a sense counteracting said 8 fluctuations during a second fluctuation period, stori ng during said second period a feedback signal representative of the changed values of said input variable while,

continuing to measure said output'variable, storing the values thereof for said interval and producing the delayed control signal therefrom, combining during a third fluetuation period the delayed control. signal with the feed back signal stored for a whole period, changing said input variable during said third periodin step with the combined signals and concurrently storing the changed values of said input variable for another whole period, continuing during said third period to measure said output variable, storing the values-thereof for said interval and producing the delayed control signal therefrom, and

proceeding in like manner during subsequent periods,

thereby eliminating the eifect of said lag.

' References Cited in the file of this patent UNITED STATES PATENTS Graef etal. Mar. 28,

' Gates et al Oct.- 25, 5, 

4. A METHOD OF OPERATING A ROTARY REFINING FURNACE COMPRISING THE STEPS OF INTRODUCING A GAS BELOW THE LEVEL OF A MELT IN SAID FURNACE TO PRODUCE AN EXHAUST GAS OF A COMPOSITION WHICH CONSTITUTES AN OUTPUT VARIABLE SUBJECT TO PERIODIC FLUCTUATIONS, SAID OUTPUT VARIABLE RESPONDING WITH A PREDETERMINED TIME LAG TO CHANGES IN AN INPUT VARIABLE DEFINING THE DEPTH AT WHICH SAID GAS IS INTRODUCED INTO SAID MELT, MEASURING SAID OUTPUT VARIABLE DURING A FIRST FLUCTUATION PERIOD WITH SAID INPUT VARIABLE MAINTAINED CONSTANT, STORING THE SO MEASURED VALUES OF SAID OUTPUT VARIABLE FOR AN INTERVAL EQUAL TO ONE WHOLE FLUCTUATION PERIOD LESS THAN A TIME EQUAL TO SAID LAG, PRODUCING AT THE END OF SAID INTERVAL A DELAYED CONTROL SIGNAL FROM THE VALUES SO STORED, CHANGING SAID INPUT VARIABLE IN STEP WITH SAID CONTROL SIGNAL AND IN A SENSE COUNTERACTING SAID FLUCTUATIONS DURING A SECOND FLUCTUATION PERIOD, STORING DURING SAID SECOND PERIOD A FEEDBACK SIGNAL REPRESENTATIVE OF THE CHANGED VALUES OF SAID INPUT VARIABLE WHILE CONTINUING TO MEASURE SAID OUTPUT VARIABLE, STORING THE VALUES THEREOF FOR SAID INTERVAL AND PRODUCING THE DELAYED CONTROL SIGNAL THEREFROM, COMBINING DURING A THIRD FLUCTUATION PERIOD THE DELAYED CONTROL SIGNAL WITH THE FEEDBACK SIGNAL STORED FOR A WHOLE PERIOD, CHANGING SAID INPUT VARIABLE DURING SAID THIRD PERIOD IN STEP WITH THE COMBINED SIGNALS AND CONCURRENTLY STORING THE CHANGED VALUES OF SAID INPUT VARIABLE FOR ANOTHER WHOLE PERIOD, CONTINUING DURING SAID THIRD PERIOD TO MEASURE SAID OUTPUT VARIABLE, STORING THE VALUES THEREOF FOR SAID INTERVAL AND PRODUCING THE DELAYED CONTROL SIGNAL THEREFROM, AND PROCEEDING IN LIKE MANNER DURING SUBSEQUENT PERIODS, THEREBY ELIMINATING THE EFFECT OF SAID LAG. 